This invention relates to a start-up control circuit for a polyphase induction motor and, more particularly, to such a circuit which reduces the rate of initial acceleration of the motor and which reduces the start-up inrush of motor current.
Polyphase induction machines such as motors are well known and have been widely used in the art. The induction motor is one in which alternating current is supplied to the stator directly and to the rotor by induction or transformer action from the stator. The rotor in these machines may be one of two types, the most popular of which is the squirrel-cage rotor wherein its winding consists of conducting bars embedded in slots in the rotor iron and short-circuited at each end by conducting end rings. The extreme simplicity and ruggedness of the squirrel-cage construction are pronounced advantages of this type of induction motor. It has been found desirable in the use of these machines to reduce the voltage to the motor during start-up and thereafter increasing to full voltage during a running condition. Reducing this voltage serves a two-fold purpose. First, a reduction in the rate of acceleration of the motor and its load. For example, in the case of the use of such a motor as a power source for a crane trolley, reducing the rate of acceleration allows for smooth acceleration and minimizing the swing of the load. Similarly, when such motors are used on a conveyor, reducing the acceleration produces a more gradual acceleration which minimizes disturbance of products on the conveyor. Secondly, reduction of the applied voltage during motor start-up reduces current inrush in the motor. This has the advantage of minimizing or avoiding the dimming of lights in the motor environment during such start-up operation; and also reduces or avoids the need for over-sizing the power distribution system to accommodate this initial inrush of start-up motor current.
Control circuits for reducing the start-up current of a squirrel cage induction motor are known and have been used in the art. However, these known circuits have the disadvantages that the control circuit is not adequately isolated from the motor control inputs and therefore switching transients associated with the motor contactor relay coil can disadvantageously affect the control circuit. Further, these known control circuits allow the motor voltage applying contactor relay contacts to be applied to the load during the motor turn-off operation and thereby allow undesirable arcing of the relay contactor contacts during at least the initial portions of the off-period operation.
These and other disadvantages are overcome by the present invention wherein there is provided a control system for polyphase induction machines which provides start-up control and which provides isolation of the input of the control system from the motor control circuits and which removes the motor operating voltage from the motor load during off-period operation.